The father of a mating pair expresses Huntington disease. The mother is healthy. What is the recurrence risk for the offspring? Draw a Punnett square to demonstrate the inheritance.
Since this is not my area of expertise, I searched Google under the key words "huntington disease genetic probability" to get this source:
Huntington's disease is inherited in an autosomal dominant fashion. The probability of each offspring inheriting an affected gene is 50%. Inheritance is independent of gender, and the phenotype does not skip generations.
Huntington's disease has autosomal dominant inheritance, meaning that an affected individual typically inherits one copy of the gene with an expanded trinucleotide repeat (the mutant allele) from an affected parent.[1] Since penetrance of the mutation is very high, those who have a mutated copy of the gene will have the disease. In this type of inheritance pattern, each offspring of an affected individual has a 50% risk of inheriting the mutant allele and therefore being affected with the disorder (see figure). This probability is sex-independent.[18]
Trinucleotide CAG repeats over 28 are unstable during replication and this instability increases with the number of repeats present.[16] This usually leads to new expansions as generations pass (dynamic mutations) instead of reproducing an exact copy of the trinucleotide repeat.[14] This causes the number of repeats to change in successive generations, such that an unaffected parent with an "intermediate" number of repeats (28–35), or "reduced penetrance" (36–40), may pass on a copy of the gene with an increase in the number of repeats that produces fully penetrant HD.[14] Such increases in the number of repeats (and hence earlier age of onset and severity of disease) in successive generations is known as genetic anticipation.[14] Instability is greater in spermatogenesis than oogenesis;[14] maternally inherited alleles are usually of a similar repeat length, whereas paternally inherited ones have a higher chance of increasing in length.[14][19] It is rare for Huntington's disease to be caused by a new mutation, where neither parent has over 36 CAG repeats.[20]
In the rare situations where both parents have an expanded HD gene, the risk increases to 75%, and when either parent has two expanded copies, the risk is 100% (all children will be affected). Individuals with both genes affected are rare. For some time HD was thought to be the only disease for which possession of a second mutated gene did not affect symptoms and progression,[21] but it has since been found that it can affect the phenotype and the rate of progression.[14][22]
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Huntington's disease is an autosomal dominant genetic disorder. This means that if one parent has the disease, there is a 50% chance of passing it on to their offspring with each pregnancy. Let's draw a Punnett square to demonstrate this inheritance pattern.
Let "H" represent the Huntington's disease allele and "h" represent the healthy allele:
| H | h |
--------------------------------
H | HH | Hh |
--------------------------------
h | Hh | hh |
In this Punnett square, the father's genotype is "Hh" (one allele for Huntington's disease and one normal allele), while the mother's genotype is "hh" (two normal alleles).
Looking at the results, we see that:
- 50% of the offspring will inherit one copy of the disease-causing allele (Hh genotype) from the father and one normal allele (h) from the mother. They will be carriers of the disease but will not express symptoms themselves.
- The other 50% of the offspring will inherit two normal alleles (hh genotype), one from each parent, making them unaffected and not carriers of the disease.
Therefore, the recurrence risk for the offspring in this case is 50%.
To determine the recurrence risk for an offspring in this scenario, we need to understand the inheritance pattern of Huntington's disease. Huntington's disease is an autosomal dominant disorder, which means that a single copy of the mutated gene is enough to cause the disease.
In this case, if the father has Huntington's disease, it means he has one copy of the mutated gene. The mother, on the other hand, is healthy, which means she does not have the mutated gene.
To demonstrate the inheritance using a Punnett square, we can represent the father as Hh (H for the dominant allele, which causes the disease, and h for the recessive allele, which is the healthy allele). The mother can be represented as hh (both recessive alleles).
| H | h |
-----------------------------
H | HH | Hh |
-----------------------------
h | Hh | hh |
Now let's interpret the Punnett square:
1. HH: Represents a child that inherits one copy of the mutated gene from the father (H) and one copy of the healthy gene from the mother (h). This child will have Huntington's disease.
2. Hh: Represents a child that inherits one copy of the mutated gene from the father (H) and one copy of the healthy gene from the mother (h). This child will have Huntington's disease.
3. Hh: Represents a child that inherits one copy of the mutated gene from the father (H) and one copy of the healthy gene from the mother (h). This child will have Huntington's disease.
4. hh: Represents a child that inherits two copies of the healthy gene, one from each parent. This child will be healthy and not have Huntington's disease.
From the Punnett square, we can see that there is a 75% chance that any given offspring will have Huntington's disease (HH, Hh, or Hh), and a 25% chance that the offspring will be healthy (hh).
Therefore, the recurrence risk for any offspring of this mating pair is 75% for Huntington's disease and 25% for being healthy.